Poedit is a free tool to translate software, websites and apps which use Gettext PO files for localization. WordPress, PHP projects and Django for example use PO files and on Linux PO files are used for nearly everything.
Poedit uses a very compact format for its editor so it is easy to use and lets you focus on translation without being distracted. It is using a TM to store your translations and therefore it is able to suggest translations in future projects.
Available for: Windows, OS X, Unix/Linux